Thermionic valve amplifying circuits



Oct. 22, 1940.

A. D. BLUMLEIN THERIIDNIC VALVE AIIPLIFYING CIRCUITS Filed llay 18, 1938INVENTOR nuw DOM-R 5117mm! BY ATTORNEY UNITED;STATES THERMIONIC VALVAMPLIFYING omourrs Alan Dower Blumlein, London, England, assignor toElectric & Musical Industries Limited, Hayes,

Middleseilr', Britain England, a. corporation 1 of Great icati y 18,1938, SerialNo. 203, 19

In Great Britain June 5, 1937 This invention relates to thermionic valveamplifying circuits and is particularly concerned With power amplifying.circuits.

It. is. usual in such circuits to employ as the last valve of the'stagea pentode valve or a power tetrode. These-valves are provided withscreening. grids which are usually maintained at a constant positivepotential with respect to the oathode. Valves of this kind give agreater output for a given anode potential than triodes, but they sufferfromthe disadvantage that their characteristic impedance to the anode isvery much greaterthan the ratio'of the steady D. 0. Voltage to thesteady D.-C. current. Consequently, in order to obtain a.goodpo weroutput the impedance of the load-must be very-much less than'thecharacteristic impedance of the valve. Furthermore, particularlyin'the-case of either a pentode or a power tetrode'valve, which iscoupled to its load'through a transformer; it "is known that if the loadis removed; thenf-for relatively moderate variations in potential of theinput grid, excessively high output voltages will be produced which maybesufficient? to cause disruption of'the valve or of the outputtransformer. 1

-It is the chief object of thepresent invention to provide an improvedcircuit in which the characteristic impedance-of valves of the'kindreferred to can' be more nearlyTmatche'd to their v optimum loads and inwhich the danger of an excessive increase of 'voltageonremoval of theload may be substantially avoided.

According-to theinvention'a thermionic valve amplifying circuit isprovided employing a pentode or tetrode valve in which the screeninggrid of the valve is so connected that the potential thereof varies inproportion to the A. C. potential variations of the anode of the valve.This result may be achieved by connecting the screening grid to atapping point on the primary winding, of an output transformer, oralternatively, the screening grid may be connected to a source ofpotential through a resistance and connected through a by-pass condenserto a tapping point on the primary winding of the output transformer.Other alternative connections for causing the potential of the screeninggrid to vary in proportion to the potential of the anode may beemployed. For example, the screening grid may be connected to a tappingpoint in a choke connecting the anode of the valve to the sourceof anodecurrent.

The effect of causing the screen to vary in potential proportional tothe variations in anode potential is to cause the valve to functionsomewhat after the manner of a triode. It is known that in atriodetheanode impedance isless than the ratio of the steady D. C. anode voltageto the steady D. Csanode current, Hence, by choosing a suitable tappingpoint for the screening grid the characteristic impedance of the anodeand screen eifectively combined can .be given any desired value betweenthe impedanceof a pentode or tetrode and the impedance of a triode. Asuitable tapping point, in practice, may be found to be such that thevoltage swing on the screen is between about a quarterand'ahalf thevoltage swing-on.the.anode. v .1 1

In order that the invention may be clearly understood and readilycarried into efiect it will now be more fully described with referenceto the accompanying drawinginwhich: i

Fig. 1' illustrates the. application of the invention to a circuitembodying apentodevalve; and,

Fig. 2 illustrates the application of the invention to a tetrode valve.As shown in Fig 1, signals to be amplified are applied betweentheicontrol grid 3 and cathode 4 of a pentode valve through a couplingcondenser 5 and leak resistance 6, the cathode of the valve being biasedby a dropper resistance 1 shunted by a decoupling condenser 8. Thesuppressor grid 9'is connected'to the-cathode in the usual manner. Theanode I0 is connected through the primary winding ll of an outputtransformer to thepositive terminal of a suitable sourceof high tensioncurrent the secondary winding I2 of the transformer being connected to aload not shown. The screening grid. 13 is connected as shown to atapping point on the primary'winding I I so that the screening grid [3varies in potential in pro portion to the potential variations on theanode l0. As stated above, the tapping point maybe arranged so that thevoltage swing on the screen grid l3 isbetween about a quarter and a halfthe voltage swing on the anode l0. 40

The arrangement shown in Fig. 2 of the drawing illustrates the inventionas applied to a tetrode valve and, in this case, signals to be amplifiedare applied between the control grid l4 and cathode l5 through an inputtransformer IS the cathode I5 being biased by a dropper resistance l1shunted by a decoupling condenser Ill. The anode l 9 is connected to thepositive terminal of a source of anode current through a choke 20 and isconnected through a by-pass condenser 2| to the primary winding 22 of anoutput transformer the secondary winding 23 of which is connected to aload, not shown.

In this example of the invention the screening grid 24 is connectedthrough a resistance 25 to the source of high tension current and isalso connected in the manner shown through a bypass condenser 26 to atapping point on the primary winding 22 of the output transformer. Ifdesired, in Fig. 2, instead of connecting the screen grid 24 to atapping point on the primary winding 22, it may be connected to asuitable tapping point on the choke 20. Many other alternativeconnections are possible.

The circuits described also have the advantage that if the loads areremoved the output voltage obtained for relatively small potentialvariations of the control grids is not as great as with the arrangementsusually employed, since the screening grid which is partially effectivein controlling the cathode current will swing in opposite phase to thegrid and so prevent or reduce an excessive output voltage.

In some cases the decoupling condensers 8-and l8 of Figs 1 and 2respectively may not be employed, in which cases negative feedback willoccur tending to reduce the second harmonic output. The effect of suchfeedback will, however, be to increase the impedance of the valves sothat in cases where it is desired to reduce the second harmonic outputit should be arranged that the potential variations of the screen shouldbe greater compared with the cases in which the decoupling condensersare employed in order to maintain the anode impedance at the requiredvalue. In each example described it will also be appreciated that thecurrent which flows in the screening grid also contributes to the usefuloutput of the circuit.

I claim:

1. In an amplifying circuit, a thermionic valve having an anode, acathode, a grid electrode and a screening electrode, an input circuitfor said valve connected between the cathode and grid electrode thereof,said input circuit including means for connecting to a source of signalvoltage, an output circuit for the valve including a load impedanceacross which is developed the output voltage, and means for connectingthe screening electrode to an intermediate point of said load impedancedevice to thereby produce a voltage swing on the screening electrodewhich is an amplitude reduced image of the voltage swing produced on theanode due to signal energy impressed upon the input circuit, the pointof connection of the screening electrode to the load impedance being sochosen that the voltage swing on the screen is of the order of thevoltage swing on the anode.

2. In an amplifying circuit, a thermionic valve having an anode, acathode, a grid electrode and a screening electrode, an input circuitfor said valve connected between the cathode and grid electrode thereof,said input circuit including means for connecting to a source of signalvoltage, an output circuit for the valve including a load impedanceacross which is developed the output voltage, and means for connectingthe screening electrode to an intermediate point of said load impedancedevice to thereby produce a voltage swing on the screening electrodewhich is an amplitude reduced image of the voltage swing produced on theanode due to signal energy impressed upon the input circuit, the loadimpedance comprising a transformer having a primary winding connected inthe output circuit of the valve and a secondary winding adapted to beconnected to a utilizing device, said screening electrode beingconnected to a point of said primary winding.

3. In an amplifying circuit a thermionic tube having an anode, acathode, a grid electrode and an auxiliary electrode, an input circuitfor said tube connected between the grid electrode and the cathodethereof and including means for connection to a source of signalvoltage, a source of anode potential for said tube including a hightension terminal, a connection including a choke coil between the anodeof the tube and said high tension terminal, an output circuit for saidtube including a load impedance connected between the anode and cathodeof the tube, means including an impedance device for connecting theauxiliary electrode to said high tension terminal and a by-passcondenser connected between said auxiliary electrode and an intermediatepoint of said load impedance whereby there is impressed upon theauxiliary electrode an amplitude reduced image of the voltage swingproduced on the anode due to signal energy impressed upon the inputcircuit.

4. An arrangement as described in the next preceding claim characterizedby that the output impedance comprises a transformer having a primarywinding connected in the output circuit of the tube and a secondarywinding provided with means for connection to a utilizing device, saidauxiliary electrode being connected through the lay-pass condenser to anintermediate point of said primary winding.

ALAN DOWER BLU'MLEIN.

